Three-dimensional measurement apparatus, three-dimensional measurement method, and three-dimensional measurement non-transitory computer readable medium

1. A three-dimensional measurement apparatus, comprising:

a projector configured to project a patterned light onto a measurement target;

a camera configured to capture an image of the measurement target onto which the patterned light is projected, with an exposure time which is predetermined; and

a processor configured to:

extract a plurality of feature points included in the image and to calculate positions of a three-dimensional point group expressing a three-dimensional shape of the measurement target based on the plurality of feature points, and

determine the exposure time such that at least one of a number of the plurality of feature points and a number of points included in the three-dimensional point group is at least a threshold that is defined based on either a maximum number of the plurality of feature points or a maximum number of points included in the three-dimensional point group, and the exposure time is shorter than an exposure time for the maximum number.

2. The three-dimensional measurement apparatus according to claim 1 ,

wherein the camera is configured to capture a plurality of the images while changing the exposure time,

the processor is further configured to estimate, based on the plurality of images, how at least one of a number of the plurality of feature points and a number of points included in the three-dimensional point group changes depending on the exposure time, and

to determine the exposure time based on an estimation result made by the processor.

3. The three-dimensional measurement apparatus according to claim 2 ,

wherein the estimation result includes:

a first stage in which the number of the plurality of feature points and the number of points included in the three-dimensional point group increase as the exposure time extends, and

a second stage that is adjacent to the first stage and in which the number of the plurality of feature points and the number of points included in the three-dimensional point group change more gently than in the first stage as the exposure time extends, and at least one of the number of the plurality of feature points and the number of three-dimensional point groups is equal to or greater than the threshold, and

the processor is configured to determine the exposure time based on a boundary between the first stage and the second stage.

4. The three-dimensional measurement apparatus according to claim 3 ,

wherein the estimation result further includes a third stage that is adjacent to the second stage and in which the number of the plurality of feature points and the number of points included in the three-dimensional point group decrease as the exposure time extends, and

an absolute value of an amount of a change in the number of the plurality of feature points and an absolute value of an amount of a change in the number of points included in the three-dimensional point group when the exposure time is changed by a predetermined amount are smaller in the second stage than in the third stage.

5. The three-dimensional measurement apparatus according to claim 3 ,

wherein the processor is configured to estimate relationships between the exposure time in the first stage and the exposure time in the second stage, the number of the plurality of feature points, and the number of points included in the three-dimensional point group, using a function including one or more parameters.

6. The three-dimensional measurement apparatus) according to claim 5 ,

wherein, when the exposure time is t and the one or more parameters are N o and τ, the processor estimates the relationships between the exposure time in the first stage and the exposure time in the second stage, the number of the plurality of feature points, and the number of points included in the three-dimensional point group, using the function N(t)=N o (1−exp(−t/τ)).

7. The three-dimensional measurement apparatus according to claim 6 ,

wherein, when the threshold is expressed as A and is greater than zero and not more than one,

the processor determines the exposure time t using t=τ ln(1/(1−A)).

8. The three-dimensional measurement apparatus according to claim 1 ,

wherein the processor is configured to determine the exposure time or a light amount of the projector such that at least one of a number of the plurality of feature points and a number of points included in the three-dimensional point group is equal to or greater than the threshold, and the exposure time is shorter than an exposure time for the maximum number.

9. A three-dimensional measurement method, comprising:

projecting a patterned light onto a measurement target;

capturing an image of the measurement target onto which the patterned light is projected, with an exposure time which is predetermined;

extracting a plurality of feature points included in the image and calculating positions of a three-dimensional point group expressing a three-dimensional shape of the measurement target based on the plurality of feature points; and

determining the exposure time such that at least one of a number of the plurality of feature points and a number of points included in the three-dimensional point group is at least a threshold that is defined based on either a maximum number of the plurality of feature points or a maximum number of points included in the three-dimensional point group, and the exposure time is shorter than an exposure time for the maximum number.

10. A three-dimensional measurement non-transitory computer readable medium, storing a program causing a processor provided in a three-dimensional measurement apparatus including a projector configured to project a patterned light onto a measurement target and a camera configured to capture an image of the measurement target onto which the patterned light is projected, with an exposure time which is predetermined, to execute operations of:

extracting a plurality of feature points included in the image and to calculate positions of a three-dimensional point group expressing a three-dimensional shape of the measurement target based on the plurality of feature points, and

determining the exposure time such that at least one of a number of the plurality of feature points and a number of points included in the three-dimensional point group is equal to or greater than a threshold that is defined based on either a maximum number of the plurality of feature points or a maximum number of points included in the three-dimensional point group, and the exposure time is shorter than an exposure time for the maximum number.